C.1 Message Distribution Patterns

Goal

Provide a procedure to handle messages that can be successfully delivered but cannot be properly processed due to invalid or corrupted message data.

Participants

Message client

Monitors a message delivery channel, receives messages, and processes correctly-formatted messages according to a set of business rules.

Malformed message channel

A messaging destination used for messages that cannot be processed.

Message server

Responsible for routing messages to a delivery channel and receiving messages from a client.

Interactions

The messaging server delivers messages to a client. If the client cannot properly process the message, it routes a copy to a special messaging destination configured within the messaging server. A human can monitor this malformed message channel and delete or act upon the messages it contains.

Notes

In JMS, use a specially designated Queue or a Topic in your messaging server as a malformed message channel. You use your message server's tools to periodically examine the channel to identify unprocessed messages. The incidence of malformed messages will probably be higher when using SMTP for messaging.

Goal

Send a message once and only once to a destination known to the sender.

Participants

Sender

Originates a message and is aware of the destination.

Recipient

Receives a message.

Messaging server

Relays a message from a sender to a recipient.

Interactions

The sender dispatches a message to a messaging server, which is responsible for relaying the message to a particular recipient. The messaging server may proactively deliver the message (by contacting the recipient directly), or hold the message until the recipient connects to retrieve it.

Notes

In a JMS environment using an enterprise message queue product, point-to-point messaging is implemented via the Queue interface. In an SMTP environment with JavaMail, all messaging is point-to-point.

Goal

Send a single copy of a message for delivery to multiple recipients who may or may not be individually known to the sender.

Participants

Sender

Originates a message of a particular type.

Messaging server

Accepts a message from a sender and delivers it to zero or more recipients.

Recipients

Register to receive (subscribe to) a particular type of message.

Interactions

Rather than identify a particular recipient, the sender identifies a message as being of a certain type. Recipients may then register with the messaging server as being interested in receiving all messages of a particular type. When a sender creates a new message, all recipients who have expressed interest in receiving messages of that type will receive a copy.

Notes

In a JMS environment, publish-subscribe is implemented via the Topic interface. In an SMTP/JavaMail environment, implementing publish-subscribe requires additional infrastructure, either within your application or within your mail server.